1. Field of the Invention
The present invention relates to receiving devices, receiving methods, and programs, and particularly to a receiving device, a receiving method, and a program that allow making of a high-precision determination as to whether or not the channel environment is a single path environment or a near delay path environment.
2. Description of the Related Art
As a modulation system for terrestrial digital broadcasting, a modulation system called an orthogonal frequency division multiplexing (OFDM) system is used.
In the OFDM system, a large number of orthogonal subcarriers are set in the transmission band. Furthermore, data is allocated to the amplitude and phase of each subcarrier and digital modulation is carried out by phase shift keying (PSK) or quadrature amplitude modulation (QAM).
The OFDM system has a characteristic that the total transmission rate thereof is similar to that of existing modulation systems although the band per one subcarrier is narrow and the transmission rate is low in the OFDM system because the whole of the transmission band is divided by the large number of subcarriers. Furthermore, the OFDM system has a characteristic that the robustness against the multipath can be enhanced by providing a guard interval to be described later.
Moreover, in the OFDM system, modulation can be carried out by inverse fast Fourier transform (IFFT) operation for performing inverse Fourier transform because data are allocated to plural subcarriers. Demodulation of an OFDM signal obtained as a result of the modulation can be carried out by fast Fourier transform (FFT) operation for performing Fourier transform.
Therefore, a transmitting device for transmitting the OFDM signal can be formed by using a circuit for performing the IFFT operation, and a receiving device for receiving the OFDM signal can be formed by using a circuit for performing the FFT operation.
Due to the above-described characteristics, the OFDM system is frequently applied to terrestrial digital broadcasting, which is highly susceptible to the influence of multipath interference. Examples of the standard of the terrestrial digital broadcasting for which the OFDM system is employed include digital video broadcasting-terrestrial (DVB-T), integrated services digital broadcasting-terrestrial (ISDB-T), and ISDB-terrestrial for sound broadcasting (ISDB-TSB).
FIG. 1 is a diagram showing OFDM symbols.
In the OFDM system, signal transmission is carried out based on the unit called the OFDM symbol.
As shown in FIG. 1, one OFDM symbol is composed of a useful symbol that is the signal interval for which the IFFT is performed at the time of transmission and a guard interval (hereinafter, referred to as GI) obtained by copying the waveform of one part in the latter half of the useful symbol. The GI is inserted at the position previous to the useful symbol on the time axis. In the OFDM system, inserting the GI can prevent interference between the OFDM symbols, which occur under a multipath environment.
If the length of the useful symbol in the OFDM symbol, i.e. the useful symbol duration as the duration that does not include the guard interval, is Tu [seconds] and the interval between subcarriers is Fc [Hz], the relationship expressed by an equation Fc=1/Tu is satisfied.
One OFDM transmission frame is formed by assembling a plurality of such OFDM symbols. For example, in the ISDB-T standard, one OFDM transmission frame is formed of 204 OFDM symbols. The insertion positions of a pilot signal are defined on the basis of the unit of this OFDM transmission frame.
In the OFDM system in which a QAM modulation system is used as the modulation system for the respective subcarriers, the amplitude and phase of the subcarrier at the time of transmission differ from those at the time of reception on each subcarrier basis due to the influence of a multipath and so on in transmission. Therefore, the receiving side needs to carry out signal equalization so that the amplitude and phase of the received signal may become equal to those of the transmitted signal.
In the OFDM system, pilot signals having predetermined amplitude and a predetermined phase are inserted in a transmission symbol in a scattered manner on the transmitting side. In addition, the receiving side obtains the frequency characteristic of the channel based on the amplitude and phase of the pilot signals to thereby equalize the received signal.
The pilot signal used to calculate the channel characteristic is referred to as the scattered pilot signal (hereinafter, referred to as the SP signal). FIG. 2 shows an arrangement pattern of the SP signals in OFDM symbols, employed in the DVB-T standard and the ISDB-T standard. In FIG. 2, the vertical direction corresponds to the time direction and the horizontal direction corresponds to the frequency direction.
FIG. 3 is a block diagram showing a configuration example of an OFDM receiver of a related art.
A tuner 2 carries out frequency conversion of an RF signal received by a receiving antenna 1 into an IF signal and outputs the IF signal to an A/D conversion circuit 3.
The A/D conversion circuit 3 executes A/D conversion for the IF signal supplied from the tuner 2 and outputs the digital IF signal to a quadrature demodulation circuit 4.
The quadrature demodulation circuit 4 performs quadrature demodulation by using a carrier supplied from a carrier generation circuit 5 to thereby acquire a baseband OFDM signal and output it. This baseband OFDM signal is a time domain signal before FFT operation.
Hereinafter, the baseband OFDM signal before the FFT operation will be referred to as the OFDM time domain signal. The OFDM time domain signal is obtained as a complex signal including a real-axis component (I-channel signal) and an imaginary-axis component (Q-channel signal) as a result of the quadrature demodulation. The OFDM time domain signal output from the quadrature demodulation circuit 4 is supplied to the carrier generation circuit 5, an FFT circuit 6, an FFT interval control circuit 7, and a delay profile estimation circuit 10.
The carrier generation circuit 5 generates a carrier having predetermined frequency based on the OFDM time domain signal supplied from the quadrature demodulation circuit 4 and outputs the carrier to the quadrature demodulation circuit 4.
The FFT circuit 6 extracts the signal in the range of the useful symbol duration from the signal of one OFDM symbol based on an FFT trigger pulse supplied from the FFT interval control circuit 7. Furthermore, the FFT circuit 6 performs the FFT operation for the extracted OFDM time domain signal to thereby extract the data carried by the quadrature modulation of the respective subcarriers.
The start position of the FFT operation is any position in the range from position A in FIG. 1, which is equivalent to a boundary of the OFDM symbol, to position B, which is equivalent to the boundary between the GI and the useful symbol. The FFT operation range is referred to as the FFT interval, and the start position of the FFT interval is specified by the FFT trigger pulse supplied from the FFT interval control circuit 7.
The FFT circuit 6 outputs an OFDM signal representing the extracted data. This OFDM signal is a frequency domain signal obtained after the FFT operation. Hereinafter, the OFDM signal obtained after the FFT operation will be referred to as the OFDM frequency domain signal. The OFDM frequency domain signal is supplied to a SP extraction circuit 8-1 and a divider circuit 8-4 in a channel distortion compensation circuit 8.
The FFT interval control circuit 7 decides the FFT interval based on the OFDM time domain signal supplied from the quadrature demodulation circuit 4 and a delay profile estimated by the delay profile estimation circuit 10, and outputs the FFT trigger pulse to the FFT circuit 6.
The channel distortion compensation circuit 8 includes the SP extraction circuit 8-1, a time direction characteristic estimation circuit 8-2, a frequency direction characteristic interpolation circuit 8-3, and the divider circuit 8-4.
The SP extraction circuit 8-1 extracts the SP signals from the OFDM frequency domain signal and removes the modulation component of the SP signals to thereby estimate the channel characteristic for the SP signals. The SP extraction circuit 8-1 outputs channel characteristic data representing the estimated channel characteristic to the time direction characteristic estimation circuit 8-2.
The time direction characteristic estimation circuit 8-2 estimates the channel characteristic for the respective OFDM symbols arranged along the time direction from the subcarrier in which the SP signal is disposed, based on the channel characteristic estimated by the SP extraction circuit 8-1. For example, by using the channel characteristic for a SP signal SP1 in FIG. 2 and the channel characteristic for a SP signal SP2, estimated by the SP extraction circuit 8-1, the time direction characteristic estimation circuit 8-2 estimates the channel characteristic for the other symbols in area A1 in FIG. 2.
The SP signal is inserted in every twelfth subcarrier in an OFDM symbol of the same time. Therefore, the channel characteristic of every third subcarrier is estimated by the time direction characteristic estimation circuit 8-2. The time direction characteristic estimation circuit 8-2 outputs data representing the estimated channel characteristic of every third subcarrier. The data output from the time direction characteristic estimation circuit 8-2 is supplied to the frequency direction characteristic interpolation circuit 8-3 and the delay profile estimation circuit 10.
The frequency direction characteristic interpolation circuit 8-3 executes frequency interpolation processing as processing of interpolating the channel characteristic in the frequency direction to thereby estimate the channel characteristic of the subcarriers for each OFDM symbol in the frequency direction from the channel characteristic of every third subcarrier.
The frequency interpolation processing is realized by applying a low pass filter to data arising from triple upsampling for the data representing the channel characteristic of every third subcarrier. The frequency direction characteristic interpolation circuit 8-3 is given plural low pass filters as the interpolation filter, and the interpolation filter used in the frequency interpolation processing is specified by a filter selection signal supplied from a frequency interpolation filter selection circuit 11. For example, the frequency direction characteristic interpolation circuit 8-3 estimates the channel characteristic for, of the positions of the OFDM symbol included in area A2 in FIG. 2, the positions for which the channel characteristic is yet to be estimated.
As a result, the channel characteristic of all of the subcarriers is estimated. The frequency direction characteristic interpolation circuit 8-3 outputs, to the divider circuit 8-4, data representing the result of the estimation of the channel characteristic of all of the subcarriers.
The divider circuit 8-4 corrects distortion included in the OFDM frequency domain signal based on the channel characteristic of all of the subcarriers, supplied from the frequency direction characteristic interpolation circuit 8-3. The divider circuit 8-4 outputs the OFDM frequency domain signal whose distortion has been corrected to an error correction circuit 9.
The error correction circuit 9 executes deinterleave processing for a signal interleaved on the transmitting side and executes processing such as depuncturing, Viterbi decoding, diffusion signal removal, and RS decoding. The error correction circuit 9 outputs the data obtained through the various kinds of processing to the subsequent-stage circuit as decoded data.
The delay profile estimation circuit 10 estimates the delay profile of the channel by obtaining the time response characteristic of the channel. For example, the delay profile estimation circuit 10 estimates the delay profile by performing IFFT for the channel characteristic estimated by the time direction characteristic estimation circuit 8-2 and executing threshold processing for the result of the IFFT. The part from which a value equal to or smaller than the threshold is obtained is regarded as a noise component, and it is determined that a path exists in the part from which a value exceeding the threshold is obtained.
The delay profile estimated by the delay profile estimation circuit 10 is supplied to the FFT interval control circuit 7 and the frequency interpolation filter selection circuit 11. As the method for estimating the delay profile, a method is also known in which the delay profile is estimated from an OFDM time domain signal by utilizing a matched filter (MF) whose tap coefficient is the GI period.
The frequency interpolation filter selection circuit 11 obtains the delay spread based on the delay profile estimated by the delay profile estimation circuit 10, and selects the interpolation filter having the filter band suitable for the delay spread. The frequency interpolation filter selection circuit 11 outputs the filter selection signal specifying the selected interpolation filter to the frequency direction characteristic interpolation circuit 8-3.
FIG. 4 is a diagram showing a configuration example of the frequency direction characteristic interpolation circuit 8-3.
As shown in FIG. 4, the frequency direction characteristic interpolation circuit 8-3 includes frequency interpolation filter circuits 8-3a0 to 8-3aN-1, and a selector circuit 8-3b. The data that is output from the time direction characteristic estimation circuit 8-2 and represents the channel characteristic of every third subcarrier is input to the frequency interpolation filter circuits 8-3a0 to 8-3aN-1. The filter selection signal output from the frequency interpolation filter selection circuit 11 is input to the selector circuit 8-3b. 
Each of the frequency interpolation filter circuits 8-3a0 to 8-3aN-1 executes the frequency interpolation processing for the data representing the channel characteristic of every third subcarrier by using the given interpolation filter, and outputs data representing the result of the frequency interpolation processing to the selector circuit 8-3b. 
In the example of FIG. 4, the frequency interpolation filter circuit 8-3a0 executes the interpolation processing by using an interpolation filter having a filter band BW0, and the frequency interpolation filter circuit 8-3a1 executes the interpolation processing by using an interpolation filter having a filter band BW1. The frequency interpolation filter circuit 8-3aN-1 executes the interpolation processing by using an interpolation filter having a filter band BW(N-1). FIG. 5 is a diagram in which the filter bands BW0 to BW3 are represented on the time axis.
In the example of FIG. 5, the bandwidth of the filter band BW0 is the largest and the bandwidth of the filter band BW3 is the smallest. The position of the upward white triangle indicates the center position of the filter band. The frequency interpolation processing is executed in such a way that the center position of the filter band is set to the same position as the center position of the delay spread.
The selector circuit 8-3b selects, from the data supplied from the frequency interpolation filter circuits 8-3a0 to 8-3aN-1, the data of the result of the interpolation of the channel characteristic, obtained by the frequency interpolation processing with use of the interpolation filter specified by the filter selection signal. The signal selected by the selector circuit 8-3b is output to the divider circuit 8-4.
Examples of documents of related arts include Japanese Patent Laid-open No. 2002-232390 and Japanese Patent Laid-open No. 2008-35377.